1. Field of the Invention
This invention relates generally to sensors that measure stresses induced on mechanical structures and surface acoustic wave (SAW) communication devices, and more particularly to a strain gauge sensor utilizing a piezoelectric xe2x80x9ctransmittingxe2x80x9d SAW correlator affixed to or embedded into a surface and a xe2x80x9creceivingxe2x80x9d SAW correlator that is not affixed to the surface which receives a complex RF waveform, wherein the application of stress alters the center frequency of the xe2x80x9ctransmittingxe2x80x9d SAW correlator, and the frequency of the complex RF output waveform and measurement of the center frequency yields an indication of the applied stress.
2. Brief Description of the Prior Art
A surface acoustic wave (SAW) filter, which is well known in the art, is typically comprised of a first set of closely-spaced, electrically conductive traces that resemble fingers (known as an interdigitated transducer), closely spaced from a second set of complimentary, interdigitated output conductive traces (known as output transducers). Both sets of fingers are etched onto the surface of a piezoelectric substrate. In SAW devices, the spacing between the interdigitated transducers affects the frequency of sensitivity.
Surface Acoustic Wave (SAW) detectors and strain gauge devices have also been proposed and implemented to some extent in the prior art; however, most of these devices have been constructed as a surface acoustic wave (SAW) filter sensitive to a particular frequency only, with no encoding.
Surface acoustic wave (SAW) transmitters having a surface acoustic wave resonator and an oscillator are also known in the art. Typically, the resonator section is comprised of a central interdigitated transducer, and a multitude of closely spaced, electrically conductive reflecting strips arranged at both sides of the interdigitated transducer. Both the interdigitated transducer and reflecting strips are deposited onto the surface of a piezoelectric substrate.
Spread Spectrum (SS) radios are well known to provide robust data transfer in varied and/or harsh radio frequency (RF). Spread spectrum (SS) radios were originally developed for military use, but are now becoming popular for commercial applications. Spread spectrum and the derivatives thereof, are quickly becoming the technology of choice for mass communications cellular based technology. Signal spreading provides significant immunity to man-made noise (intentional or unintentional) as well as a reduced probability of interference with existing equipment.
Another important benefit is that spread spectrum (SS) systems offer significant immunity to RF multipath. This is critical for transceivers located in or inside metal objects (i.e. within air frames or space frames). Multipath refers to multiple signal paths due to reflections that may exist between a transmitter and receiver. A classic example of multipath is the xe2x80x9cghost imagesxe2x80x9d, which commonly exist on a standard TV pictures. Multipath occurs when path length differences are exactly (n+xc2xd) wavelength. The resulting cancellation can be almost complete for narrow band transmissions. A spread spectrum (SS) signal, which exists over a large frequency or equivalently large wavelength range, may be partially canceled, but other portions will still be received. Since a SS signal contains significant redundancy, complete data can still be extracted from the resulting signal.
Piezoelectric materials are a unique family of materials that can be deformed by an applied electric field, and conversely, a deformation of the material produces an electric field. An antenna will reflect RF energy that is transmitted at its nominal frequency. Further, stress-induced deformation of a SAW-based sensor attached to an antenna will shift the prime frequency response causing the greatest energy to be reflected at a frequency slightly different from the nominal frequency. Therefore, an electric field imposed on the fingers causes a local deformation that launches a wave on the surface of the substrate. This surface wave can be converted back to an electrical signal by being read off another set of interdigitated transducers.
Reeder et al, U.S. Pat. No. 3,978,731 discloses a surface acoustic wave transducer wherein SAW wave are propagated across a thin, flexible diaphragm which is subjected to an applied external pressure. Since the wave velocity and path length vary with diaphragm deformation, the acoustic wave delay time is a function of the applied external pressure. Electroacoustic transducers are fabricated on opposite edges of the diaphragm for electronic excitation and detection of the surface acoustic wave. An electronic feedback path including the two transducers, the wave path, and an electronic amplifier oscillates at a frequency which is determined by the delay time required for acoustic wave propagation over the diaphragm path, and which decreases approximately linearly with applied external pressure. A second acoustic path called the reference path has a length equal to the first path and in the preferred embodiment contains a diaphragm which is subject to a different applied pressure. A second electronic feedback path composed of two transducers, the reference path and a second amplifier oscillates at a second frequency called the reference frequency. By applying a sample of the first and second oscillator voltages to a semiconductor mixer, a difference frequency output is obtained which is proportional to the differential pressure. The difference frequency output is approximately independent of temperature, and is converted to various digital codes by use of standard frequency counter circuits. If only one set of transducers is used, the device can also measure temperature in a digital manner. Stress or strain measurements may also be made by bonding the diaphragms or only the transducers directly on the physical surface to be measured.
Ebata, U.S. Pat. No. 4,249,418 discloses a temperature detector having a transmitting section including an oscillator which has a surface acoustic wave resonator that includes a Y-cut and Z-propagation piezoelectric base plate, of which the frequency characteristic varies responding to the temperature of the base plate, the oscillator generating an oscillation output of a frequency corresponding to the frequency characteristic and an antenna for transmitting the oscillation output. A receiving section includes a receiving antenna, a means for detecting the output of the receiving antenna, and a signal processing circuit for processing the output of the detecting means to generate at least one of the temperature display and control signals of the base plate of the surface acoustic wave device.
Mariani et al, U.S. Pat. No. 5,823,425 discloses a surface acoustic wave (SAW) sensing device for remotely sensing structural integrity of a physical structure. The sensing device includes a piezoelectric substrate with a notch formed part way in the bottom of the substrate and along the width thereof. The substrate is mounted to a physical structure. An antenna is coupled to the RF circuit on the substrate and is capable of receiving and transmitting a RF signal. Interdigital input and output transducers are disposed on the upper surface of the substrate. The input transducer is located adjacent one end of the substrate and the output transducer is located adjacent an opposing end of the substrate. Bus bars connect the input and output transducers. The input transducer provides a complementary first response upon receipt of an RF expanded linear/nonlinear FM signal from the antenna and transmits this compressed pulse to the output transducer. The output transducer provides a second response upon receipt of the first response and transmits the same to the antenna via the bus bar. When the substrate is strained beyond a predetermined critical level, the substrate is fractured along the notch and the first response emitted by the input transducer is prevented from being transmitted to the output transducer indicating SAW sensor failure and that structural integrity has been compromised.
Ruile et al, U.S. Pat. No. 6,084,503 discloses Radio-interrogated surface-wave technology sensor, in which the sensitive element is an impedance which is electrically connected as termination to a surface-wave structure of the sensor.
The present invention is distinguished over the prior art in general, and these patents in particular by a passive SAW strain sensor for remotely sensing strain within a surface that has a receiving SAW correlator with a signal input interdigitated transducer and a plurality of adjacent successive spaced interdigitated output transducers on a piezoelectric substrate, and a transmitting SAW correlator having a plurality of adjacent successive spaced interdigitated output transducers and a signal output interdigitated transducer on a piezoelectric substrate. The transmitting SAW correlator is affixed to or embedded in the surface such that its substrate will be stressed when the surface is strained and the receiving SAW correlator is not affixed to or embedded in the surface. The output transducers of the receiving SAW correlator and the transmitting SAW correlator are spatially placed to define a unique frequency of sensitivity, and their outputs are coupled together. A complex RF waveform signal applied to the receiving SAW correlator is transformed into an acoustic wave having a distinct output pulse that supplies power to the output transducers of the transmitting SAW correlator which is transformed into a complex RF output waveform. The application of stress on the transmitting SAW correlator shifts its center frequency proportionally and changes the frequency of the complex RF output waveform, and measurement of the center frequency change yields an indication of the applied stress.
It is therefore an object of the present invention to provide a passive SAW strain sensor for remotely sensing strain within a surface wherein the excitation signal and the readout signal will is a complex RF waveform, which will reduce noise and allow for differentiation between multiple devices in close proximity.
It is another object of this invention to provide a passive SAW strain sensor for remotely sensing strain within a surface that is extremely small does not require batteries or power-scavenging circuitry, and does not require wires or fibers to be run out from the surface being monitored.
Another object of this invention is to provide a passive SAW strain sensor for remotely sensing strain within a surface that can be interrogated from a distance using an RF signal, and a single interrogator can acquire strain data from multiple sensors.
Another object of this invention is to provide passive SAW strain sensors for remotely sensing strain within a surface that are particularly useful for non-destructive evaluation (NDE) of composite (and other) structures, which often have widely varied strength values for identical test coupons.
Another object of this invention is to provide passive SAW strain sensors for remotely sensing strain within a surface that are suited for use in non-destructive evaluation of composite materials such as composite airframes and bonding materials, and for use in inflatable bladders and airlocks, use as temperature sensors, pressure sensors, accelerometers, chemical or contamination sensors, non-contact torque sensors, automobile or aircraft non-contact tire pressure/temperature/tread measurements, implanted subcutaneous biological sensors for medical monitoring and diagnosis, and various extreme temperature range sensors.
A further object of this invention is to provide a passive SAW strain sensor for remotely sensing strain within a surface that utilizes Direct Sequence Spread Spectrum (DSSS) transmission technology which allows spectral reuse by utilizing different chipping sequences for different communication channels whereby multiple sets of sensors can be simultaneously communicating using the same band of frequencies.
A still further object of this invention is to provide a passive SAW strain sensor for remotely sensing strain within a surface that is simple in construction, inexpensive to manufacture, and rugged and reliable in operation.
Other objects of the invention will become apparent from time to time throughout the specification and claims as hereinafter related.
The above noted objects and other objects of the invention are accomplished by a passive SAW strain sensor for remotely sensing strain within a surface that has a receiving SAW correlator with a signal input interdigitated transducer and a plurality of adjacent successive spaced interdigitated output transducers on a piezoelectric substrate, and a transmitting SAW correlator having a plurality of adjacent successive spaced interdigitated output transducers and a signal output interdigitated transducer on a piezoelectric substrate. The transmitting SAW correlator is affixed to or embedded in the surface such that its substrate will be stressed when the surface is strained and the receiving SAW correlator is not affixed to or embedded in the surface. The output transducers of the receiving SAW correlator and the transmitting SAW correlator are spatially placed to define a unique frequency of sensitivity, and their outputs are coupled together. A complex RF waveform signal applied to the receiving SAW correlator is transformed into an acoustic wave having a distinct output pulse that supplies power to the output transducers of the transmitting SAW correlator which is transformed into a complex RF output waveform. The application of stress on the transmitting SAW correlator shifts its center frequency proportionally and changes the frequency of the complex RF output waveform, and measurement of the center frequency change yields an indication of the applied stress.